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F. A. Brooks

Abstract

Contrary to the usual assumption that shear stress is constant with height for a short distance above the ground, wind-tunnel tests with change in floor roughness by Jacobs (1939) and with flow over change of level by Tani (1957) show a long carry-over of irregular shear-stress profile. Such irregularity produces distortions in velocity and temperature profiles as a function of distance from the upwind discontinuity in surface roughness. Shear-stress irregularity, when Jacobs' tests are re-interpreted in per cent transition, is found to reach a distance downwind of more than 100 times the height of the roughness elements located before the prepared smooth ground, the delay in transition increasing with height above ground. Excluding large-scale vertical circulations, the ultimate eddy condition near the top of masts seems to depend on the the average roughness of the surface for several miles upwind, Since there is change with horizontal distance, thorough interpretation of vertical profiles will require consideration of horizontal gradients.

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F. A. Brooks

There have been uncontradicted reports of large altimeter errors in the vicinity of high mountains. A brief survey of pressure distributions over an airfoil with flaps shows a maximum pressure drop below static pressure of twice the velocity head. Applying this ratio to a 14,000-foot mountain in a 100-mph wind a maximum error of 700 feet is indicated. This is important, but not enough to explain the occasional reports of 2 to 3,000-foot errors. Pressure drops of this magnitude exist in tropical cyclones, and even greater depression is known in tornadoes. The pressure drop at the ground surface is seen to have an axial connection with the natural low pressure aloft. The strength of the vortex is shown to depend on the outside tangential input by the wind where the whirl velocity can be very moderate, and the superspeed spin inside a vortex is shown to be dependent on radial inflow of air which is discharged along the vortex axis. Procedures are suggested for locating mountain tornadoes and thorough investigation urged so that the great hazards of mountain vortices in a strong wind will become generally known.

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F. A. Brooks

Abstract

The complex physical meanings of multiple spectral-line absorption and reradiation by nearer air molecules can be represented satisfactorily by block-spectral bands within the Planckian curves of spectral intensity distribution.

Experimental determination of whole-spectrum, long-wave atmospheric transmissivity by parallel-beam radiometers permits the construction of an empirical, graphical integration chart. By this means, the half-space atmospheric radiation can be readily determined for ordinary air from soundings of temperature and moisture, if the magnitudes of these two factors are known in detail close to the flux plane.

For a characteristic polar-continental air mass there is no need to separate the CO2 from the H2O radiation, as called for by earlier charts.

The complex problem of the reflection of spectral-band radiation by a “gray” surface is evaluated by an inverted integration chart, including the estimation of absorption of reflected radiation by overlying air. The supplementation of sub-black emissive power of the ground by reflected atmospheric radiation obscures the meaning of upward radiation measured facing the ground.

The Gier and Dunkle aspirated black-plate radiometer can be used on clear nights to measure the effective thermal transmissivity of the atmosphere and should prove helpful in making frost forecasts, particularly in polluted atmospheres and under cirrus clouds. A severe radiation frost is found to result from an increase of about 25 per cent in atmospheric transmissivity with the arrival of dry, polar-continental air.

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John R. Goss and F. A. Brooks

Abstract

Magnitudes of the constants in Brunt's empirical expression for nocturnal radiation were determined from continuous automatic recordings of atmospheric radiation in California under cloudless skies and interpreted for the humidity at 1400 hours on the previous day. With these constants, useful estimates of the average nighttime atmospheric radiation rate can be computed from the local 1400-hours vapor pressure and average nocturnal air temperature as reported by all first-order U. S. Weather Bureau stations. With careful interpretation of weather records, the probable error will be about 3 per cent. Comparisons are made with Loennquist's formula and with calculations by atmospheric radiation charts from known soundings for extreme conditions leading to radiation frosts in southern California. Limited observations of daytime atmospheric radiation indicate that there is little difference in the average value of the radiation ratio between night and day when the character of the overhead air mass is not changing.

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Sergey Y. Matrosov, Roger F. Reinking, Robert A. Kropfli, Brooks E. Martner, and B. W. Bartram

Abstract

An approach is suggested to relate measurements of radar depolarization ratios and aspect ratios of predominant hydrometeors in nonprecipitating and weakly precipitating layers of winter clouds. The trends of elevation angle dependencies of depolarization ratios are first used to distinguish between columnar-type and plate-type particles. For the established particle type, values of depolarization ratios observed at certain elevation angles, for which the influence of particle orientation is minimal, are then used to estimate aspect ratios when information on particle effective bulk density is assumed or inferred from other measurements. The use of different polarizations, including circular, slant-45° linear, and two elliptical polarizations, is discussed. These two elliptical polarizations are quasi-circular and quasi-linear slant-45° linear, and both are currently achievable with the National Oceanic and Atmospheric Administration Environmental Technology Laboratory’s Ka-band radar. In comparison with the true circular and slant-45° linear polarizations, the discussed elliptical polarizations provide a stronger signal in the “weak” radar receiver channel; however, it is at the expense of diminished dynamic range of depolarization ratio variations. For depolarization measurements at the radar elevation angles that do not show much sensitivity to particle orientations, the available quasi-circular polarization provides a better depolarization contrast between nonspherical and spherical particles than does the available quasi-linear slant-45°polarization. The use of the proposed approach is illustrated with the experimental data collected during a recent field experiment. It is shown that it allows successful differentiation among pristine planar crystals, rimed planar crystals, long columns, blocky columns, and graupel. When a reasonable assumption about particle bulk density is made, quantitative estimates of particle aspect ratios from radar depolarization data are in good agreement with in situ observations. Uncertainties of particle aspect ratios estimated from depolarization measurements due to 0.1 g cm−3 variations in the assumed bulk density are about 0.1.

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F. A. Brooks, C. Lorenzen Jr., and L. M. K. Boelter
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Sergey Y. Matrosov, Robert A. Kropfli, Roger F. Reinking, and Brooks E. Martner

Abstract

Model calculations and measurements of the specific propagation and backscatter differential phase shifts (K DP and δ o, respectively) in rain are discussed for X- (λ ∼ 3 cm) and Ka-band (λ ∼ 0.8 cm) radar wavelengths. The details of the drop size distribution have only a small effect on the relationships between K DP and rainfall rate R. These relationships, however, are subject to significant variations due to the assumed model of the drop aspect ratio as a function of their size. The backscatter differential phase shift at X band for rain rates of less than about 15 mm h−1 is generally small and should not pose a serious problem when estimating K DP from the total phase difference at range intervals of several kilometers. The main advantage of using X-band wavelengths compared to S-band (λ ∼ 10–11 cm) wavelengths is an increase in K DP by a factor of about 3 for the same rainfall rate. The relative contribution of the backscatter differential phase to the total phase difference at Ka band is significantly larger than at X band. This makes propagation and backscatter phase shift contributions comparable for most practical cases and poses difficulties in estimating rainfall rate from Ka-band measurements of the differential phase.

Experimental studies of rain using X-band differential phase measurements were conducted near Boulder, Colorado, in a stratiform, intermittent rain with a rate averaging about 4–5 mm h−1. The differential phase shift approach proved to be effective for such modest rains, and finer spatial resolutions were possible in comparison to those achieved with similar measurements at longer wavelengths. A K DPR relation derived for the mean drop aspect ratio (R = 20.5K0.80DP) provided a satisfactory agreement between rain accumulations derived from radar measurements of the differential phase and data from several nearby high-resolution surface rain gauges. For two rainfall events, radar estimates based on the assumed mean drop aspect ratio were, on average, quite close to the gauge measurements with about 38% relative standard deviation of radar data from the gauge data.

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Paul J. Neiman, Gary A. Wick, F. Martin Ralph, Brooks E. Martner, Allen B. White, and David E. Kingsmill

Abstract

An objective algorithm presented in White et al. was applied to vertically pointing S-band (S-PROF) radar data recorded at four sites in northern California and western Oregon during four winters to assess the geographic, interannual, and synoptic variability of stratiform nonbrightband (NBB) rain in landfalling winter storms. NBB rain typically fell in a shallow layer residing beneath the melting level (<∼3.5 km MSL), whereas rainfall possessing a brightband (BB) was usually associated with deeper echoes (>∼6 km MSL). The shallow NBB echo tops often resided beneath the coverage of the operational Weather Surveillance Radar-1988 Doppler (WSR-88D) scanning radars yet were still capable of producing flooding rains.

NBB rain contributed significantly to the total winter-season rainfall at each of the four geographically distinct sites (i.e., 18%–35% of the winter-season rain totals). In addition, the rainfall observed at the coastal mountain site near Cazadero, California (CZD), during each of four winters was composed of a significant percentage of NBB rain (18%–50%); substantial NBB rainfall occurred regardless of the phase of the El Niño–Southern Oscillation (which ranged from strong El Niño to moderate La Niña conditions). Clearly, NBB rain occurs more widely and commonly in California and Oregon than can be inferred from the single-winter, single-site study of White et al.

Composite NCEP–NCAR reanalysis maps and Geostationary Operational Environment Satellite (GOES) cloud-top temperature data were examined to evaluate the synoptic conditions that characterize periods of NBB precipitation observed at CZD and how they differ from periods with bright bands. The composites indicate that both rain types were tied generally to landfalling polar-cold-frontal systems. However, synoptic conditions favoring BB rain exhibited notable distinctions from those characterizing NBB periods. This included key differences in the position of the composite 300-mb jet stream and underlying cold front with respect to CZD, as well as notable differences in the intensity of the 500-mb shortwave trough offshore of CZD. The suite of BB composites exhibited dynamically consistent synoptic-scale characteristics that yielded stronger and deeper ascent over CZD than for the typically shallower NBB rain, consistent with the GOES satellite composites that showed 20-K warmer (2.3-km shallower) cloud tops for NBB rain. Composite soundings for both rain types possessed low-level potential instability, but the NBB sounding was warmer and moister with stronger low-level upslope flow, thus implying that orographically forced rainfall is enhanced during NBB conditions.

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W. J. Williams, J. N. Brooks, D. G. Murcray, F. H. Murcray, P. M. Fried, and J. A. Weinman

Abstract

Infrared emission spectra were measured in the stratosphere at various altitudes and from various zenith angles by means of a balloon-borne Czerny-Turner spectrometer. The equation of radiative transfer was applied to the radiances measured at 11.2μ to yield a concentration profile of HNO3 vapor. The resulting HNO3 concentration profile was characterized by a negligible concentration below 14 km, a maximum concentration of ∼(1.5±0.5)×1010 molecules cm−3 at ∼(19±5) km, and a diminishing concentration above these altitudes.

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G. M. Martin, S. F. Milton, C. A. Senior, M. E. Brooks, S. Ineson, T. Reichler, and J. Kim

Abstract

The reduction of systematic errors is a continuing challenge for model development. Feedbacks and compensating errors in climate models often make finding the source of a systematic error difficult. In this paper, it is shown how model development can benefit from the use of the same model across a range of temporal and spatial scales. Two particular systematic errors are examined: tropical circulation and precipitation distribution, and summer land surface temperature and moisture biases over Northern Hemisphere continental regions. Each of these errors affects the model performance on time scales ranging from a few days to several decades. In both cases, the characteristics of the long-time-scale errors are found to develop during the first few days of simulation, before any large-scale feedbacks have taken place. The ability to compare the model diagnostics from the first few days of a forecast, initialized from a realistic atmospheric state, directly with observations has allowed physical deficiencies in the physical parameterizations to be identified that, when corrected, lead to improvements across the full range of time scales. This study highlights the benefits of a seamless prediction system across a wide range of time scales.

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